Solid-state storage devices and the block layer

the issue is that you have to erase large chunks (on the order of 128K bytes), if you are then writing in small chunks (say the 512 byte sectors that are the default, or even the 4K byte filesystem blocks) you can't just erase just before writing.

you also have the problem that erasing takes a significant amount of time and power to accomplish, so you don't want to wait until you need to erase to do so and you don't want to erase when you don't need to and are on battery